Peristaltic pumps are volumetric pumps which progressively compress a flexible tube to propel liquid along the tube under the influence of rotating members which contact the tube at spaced-apart locations. Such pumps are commonly used in cardiovascular surgery for circulating blood between a patient and a heart-lung machine. Other common uses for such pumps are the transfer of blood between a patient and a kidney dialyser and the intravenous infusion of medication.
Known advantages of peristaltic pumps include their simple construction and their containment of the pumped liquid in a simple, chemically-inert tube that can be easily sterilized. Disadvantages of known peristaltic pumps include their ability to pump gases, as well as liquids, when only the passage of liquids is desired. For example, when used in cardiovascular surgery for circulating blood between a patient and a heart-lung machine, a peristaltic pump can propel air, as well as blood that may be within the tubing, towards the patient. The risks of systemic and coronary air embolisms are well documented. U.S. Pat. No. 4,515,589 describes at column 7, starting at line 11, an inlet valve 100 designed to prevent the entrainment of air when a peristaltic pump is pumping blood into a patient.
Systems for circulating blood between a patient and a heart-lung machine generally consist of two blood circuits. A major circuit receives blood draining from the vena cavae into the right side of the heart and oxygenates and returns the blood to the patient's aorta for further transmission to the patient's vital organs and appendages. A smaller suction circuit sucks blood from the left side of the heart. This sucked blood is mainly coronary, Thebesian and bronchial return and can be rather substantial. The blood sucked from the left side of the heart is saved and returned to the major circuit for oxygenation and eventual return to the patient.
The smaller suction circuit is known to be particularly susceptible to hemolysis due, inter alia, to the forcible suction of the blood from the left side of the heart and the mixing of air with the blood. Hemolysis is the damage of red blood cells with consequent elevation of free plasma hemoglobin and the attendant threat to the kidneys. Additionally, the smaller suction circuit can actually damage the heart tissue by pulling this tissue into the sucker. It is known to minimize this damage by providing a separate valve in the suction circuit that can be opened at a predetermined negative pressure to draw additional air, rather than heart tissue, into the suction circuit. As previously noted, however, the mixing of air with the blood can cause hemolysis.
Efforts have also been made in the past to minimize the negative pressures generated by the pumping action of a peristaltic pump in the major circuit. One example is an intermediate, gravity-fed, reservoir system as described in said U.S. Pat. No. 4,515,589. There, venous return is drained into a reservoir in the major circuit. An outlet from the reservoir is connected to a peristaltic pump header. The peristaltic pump header is a double lumen device with an inner tube that opens and closes in response to a positive fluid pressure from the reservoir. As shown in FIG. 5 and FIG. 6 of the patent, it is contemplated that the inner tube will progressively collapse as the level in the reservoir drops, so that entrainment of air into the patient's system is minimized. To further avoid the entrainment of air, it is said that the inlet valve previously referred to can be added to the inlet to the pump header.
It is believed that a separate inlet valve can cause problems. For one thing, the inlet valve can be forgotten by the attending medical personnel; there is nothing to ensure that the inlet valve will be added by the attending medical personnel. For another, the separate inlet valve can contribute to hemolysis, since the blood must move between two separate components.
Even if the inlet valve were an integral portion of the pump header described in said U.S. Pat. No. 4,515,589, its employment could be ineffectual. More particularly, the direction of the pump can typically be reversed by an inadvertent flip of a switch. Further, there is nothing to prevent the backwards reception of the pump header within the pump. In either case, the inlet valve would be effectively placed on the outlet side of the pump, thereby negating its effectiveness.